In the production of glass fibers, a melter is frequently used wherein glass cullet is added at the top thereof and heated to melting temperature to form molten glass. The molten glass moves downward within the melter and then flows into the bushing. The molten glass exits the bushing through a tip plate in the form of glass fibers or filaments. The tip plate is an apparatus having a plurality of very small orifices or tips through which a plurality of glass fibers, still in molten condition, are pulled. Thereafter, the fibers are typically heat quenched and attenuated, coated with a sizing composition, and then gathered or wound onto a spool for later use.
In another apparatus commonly used to produce glass fibers, the glass cullet or batch is melted in a large furnace and the resulting molten glass flows through channels, to forehearths. The forehearths have holes in the bottom thereof through which the molten glass will flow. Beneath each hole is a flow block, a bushing block, a bushing, and a tip plate through which the molten glass is drawn to form glass fibers or filaments. The resulting fibers are then processed in the manner discussed above.
As the market for glass fibers has increased, larger volume equipment for fiber production has been developed. In modern glass fiber production facilities, it is not uncommon to attempt to pull up to, or even in excess of, 1800 glass fibers simultaneously through a single bushing. However, increasing the size of the bushing tip plates to accommodate more fibers has led to unforeseen problems. For example, to obtain proper attenuation of the glass fibers, it is imperative that they be of substantially equal temperatures as they are pulled from the bushing tip plate. Accordingly, it is important that the layer of molten glass directly above the tip plate be at a substantially uniform temperature.
However, bushings, by their very nature, tend to have varying temperature zones across their surface. Generally, the center of the bushing is hotter, as much as 100.degree. F. for example, than the outer regions, such as along the walls of the bushing. Additionally, such temperature differentials often increase as the size of the tip plate increases. Accordingly, obtaining a substantially uniform glass temperature across the bushing tip plate to obtain good fiber attenuation becomes much more difficult as the size of the tip plate increases.
Also, yardage (output) control is reduced if the fibers are of different temperatures as they are pulled from the tip plate. The fibers across the tip plate are pulled therefrom at a uniform rate. However, if the temperature of the molten glass across the tip plate varies, even as little as 100.degree. F. the pulling speed may be too fast for fibers emerging from cooler regions, causing breakage. On the other hand, fibers emerging from hotter regions may sag if the pulling speed is too slow. Thus, the existence of temperature differentials across the tip plate significantly complicates yardage (output) control of the fibers being pulled. Further, because of the breakage noted above, the number of glass fibers being pulled from a given tip plate is often not maximized.
Various production techniques have been used in attempts to equalize the molten glass temperature at the tip plate. For example, it is known to heat the tip plate by passing an electric current therethrough. However, this adds heat to all the molten glass on the tip plate. Thus, the molten glass regions initially hotter than other regions across the tip plate typically remain relatively hotter than the other, cooler regions, since both hotter and cooler regions are heated simultaneously.
Also, it is known to heat the glass in the bushing block. However, this has not proved to be entirely satisfactory as the glass flowing along the walls of the bushing block is heated more than the glass in the center of the flow passage. Thus, although the resultant additional heating is not evenly distributed throughout the molten glass mass, it typically does not totally eliminate the uneven heating pattern therein.
Further, bushing blocks with multiple flow passages, as disclosed in U.S. Pat. No. 4,264,348, have been used to enhance mixing of the glass as it flows through the bushing block. However, even this measure has not been fully capable of effectively delivering a mass of molten glass to a tip plate at a substantially uniform temperature thereacross. Accordingly, a need for a simple, but effective means for homogenizing the glass temperature across the bushing has remained.